在一个腔中N个原子纠缠态的有效制备方案

本文提出一个方案以通过探测光子的衰减来制备囚禁在一个腔中的若干个原子的W态。在我们的方案中，腔的衰减率远大于原子与腔的耦合强度。这样,对腔品质因子的要求被大大降低。这在实验上是很重要的。本方案的另一个优点是通过拉曼跃迁原子总是处于两个基态，因而原子的自发辐射也被抑制。     

Establishment of Entanglement for Two Atoms Trapped in Two Distant Bad Cavities

A scheme is presented for generation of entangled states for two atoms trapped in two distant bad cavities. The scheme can work with bad cavities with the coupling strength smaller than the cavity decay rate, which is important from the viewpoint of experiment. In the scheme the atoms have no probability of being populated in the excited state and thus the atomic spontaneous emission is suppressed, which increases the probability of success. The fidelity of the entangled state is not affected by the detection eflciency. Furthermore, the scheme does not require the detection of the left-polarized photon and right-polarized photon at the same time.     

Robust Scheme for Entangling Two Distant Atoms

A robust scheme is presented for realizing entangled states for two atoms trapped in separate cavities connected by an optical fiber. The first atom is initially in a superposition of the excited state and an auxiliary ground state not coupled to the first cavity, while the second one is initially in the ground state coupled to the second cavity. The scheme involves two atom-cavity-fiber interactions accompanied by the monitoring of the cavity decay and atomic spontaneous emission. The two atoms evolve to an entangled state through exchanging an excitation after the first interaction. The states with the excitation failing to be transferred are eliminated when a photon is detected during the second interaction. Therefore, the scheme is insensitive to the decoherence effect and detection inefficiency.     

Robust generation of high-fidelity entangled states for multiple atoms

A scheme is presented for generating entangled states of multiple atoms in a cavity. In the scheme the atoms simultaneously interact with a cavity mode, with the first atom driven by two classical fields and the other atoms driven by a classical field. Our scheme is valid even if the cavity decay rate is larger than the effective coupling strength, which is important for experiment. The generation of entangled states is conditional on the detection of a photon decaying from the cavity and thus the fidelity of the entangled state is insensitive to the detection inefficiency. Furthermore, the scheme can be applied to the case with any number of atoms in principle.     

腔QED中超导量子干涉仪概率克隆机的物理实现

提出一个量子概率克隆机的物理实现方案,该方案首先将高Q腔中的两个超导量子干涉仪分别作为初始比特和目标比特,腔模作为测量比特,通过腔模和经典微波脉冲与超导量子干涉仪的多种相互作用实现量子概率克隆机的幺正演化;然后将腔模态映射到另一个超导量子干涉仪上,通过对该超导量子干涉仪磁通量的测量完成状态坍缩,从而以最优的成功概率实现量子态的精确克隆.本方案采用双光子拉曼共振过程加快单比特门的操作速率,并且总操作时间远小于自发辐射和腔模衰变时间,因而在实验上是可行的.     

Scheme for Controlled Dense Coding via Cavity Decay

A scheme for controlled dense coding via cavity decay is proposed. In the scheme, two degenerate ground states of six-level atoms are used as the storage qubits and the leaky photons act as flying qubits. The system is robust against atomic spontaneous emissions and decoherence of cavity field. And the successful probability is nearly 1 with quantum nondemolition parity detectors and photon detectors, The scheme may be realized based on current technologies.     

Scheme for Controlled Dense Coding via Cavity Decay

A scheme for controlled dense coding via cavity decay is proposed. In the scheme, two degenerate ground states of six-level atoms are used as the storage qubits and the leaky photons act as flying qubits. The system is robust against atomic spontaneous emissions and decoherence of cavity field. And the successful probability is nearly 1 with quantum nondemolition parity detectors and photon detectors. The scheme may be realized based on current technologies.     

Teleportation via decay

We present a rare example of a decay mechanism playing a constructive role in quantum information processing. We show how the state of an atom trapped in a cavity can be teleported to a second atom trapped in a distant cavity by the joint detection of photon leakage from the cavities. The scheme, which is probabilistic, requires only a single three level atom in a cavity. We also show how this scheme can be modified to a teleportation with insurance.     

Implementing W States and an Ancilla-Free 1→M Phase-Covariant Cloning via Adiabatic Evolution

A scheme is proposed for generating maximally multi-atom W states and realizing an ancilla-free 1→M phase-covariant quantum cloning. Compared to the previous scheme, our scheme is based on performing an adiabatic evolution through three separated cavities which remain in their respective vacuum states during the whole operation, the process is much more simple, and operations are reduced relatively. During the process of cloning, our scheme is not necessary to perform a measurement on atoms. And our scheme is robust for atomic spontaneous emission, cavity decay and photon leakage out of the fiber. All these convenient places make our scheme more feasible.     

利用量子跳跃方法制备缠结离子态

提出了一种制备缠结离子态的方案,该方案是基于量子跳跃方法.当腔的衰减波不断地检测时,根据探测到的光子数,我们可以获得缠结离子态.     

Generation of multi-atom W states via Raman transitionin an optical cavity

A simple scheme is proposed to generate the W state of N Λ-type neutral atoms trapped in an optical cavity via Raman transition. Conditional on no photon leakage from the cavity, the N-qubit W state can be prepared perfectly by turning on a classical coupling field for an appropriate time. Compared with the previous ones, our scheme requires neither individual laser addressing of the atoms, nor demand for controlling N atoms to go through an optical cavity simultaneously with a constant velocity. We investigate the influence of cavity decay using the quantum jump approach and show that the preparation time decreases and the success probability increases with atom number because of a collective enhancement of the coupling.     

Robust Generation of Three-Particle W State with Atoms Trapped in Separate Cavities

This paper presents a scheme for generating three-particle W state of remote atoms trapped in leaky cavities.The scheme uses cavity decay to inject photons into a setup of optical devices which consist of a series of beam splitters and photon detectors.Photon detection on the output mode projects the atomic state into the W state.In the condition of “weakly driven approach”,it shows that the scheme is robust and has high fidelity.It also points out that the scheme is scalable to generate multi-atomic W state.     

Quantum state transfer between distant atoms via selectivity photon emission and absorption progresses

We proposed a simple scheme to deterministically generate three-dimensional (3D) quantum state transfer (QST) between two spatially atoms based on the selectivity photon emission and absorption progresses. In the present scheme, two M-type five-level atoms are trapped into two cavities connected by a fiber. By quantitatively discussing the evolution of system, we show that the effects of atom's spontaneous decay and photon leakage out of fiber can be suppressed in our scheme due to the presence of virtual excited processes in atom and fiber modes. Moreover, we also show that the present scheme can be extended to realize QST between distant nodes in a coupled array of optical cavity, which is very useful for the progress of the quantum information network.     

Dissipative generation for steady-state entanglement of two transmons in circuit QED

We present a dissipative scheme to generate an entangled steady-state between two superconducting transmon qutrits separately embedded in two coupled transmission line resonators in a circuit quantum electrodynamics(QED) setup. In our scheme, the resonant qutrit-resonator interaction and photon hopping between resonators jointly induce asymmetric energy gaps in the dressed state subspaces. The coherent driving fields induce the specific dressed state transition and the dissipative processes lead to the gradual accumulation in the population of target state, combination of both drives the system into a steady-state entanglement. Numerical simulation shows that the maximally entangled state can be produced with high fidelity and strong robustness against the cavity decay and qutrit decay, and no requirements for accurate time control. The scheme is achievable with the current experimental technologies.     

Controllable Photon Transport in a Three-Mode Optomechanical System with the Non-Rotating Wave Approximation Effect

We propose a scheme for realizing controllable photon transport in a three-mode optomechanical system comprising one cavity and two mechanical modes. We found that the non-rotating wave approximation effect can cause the ideal optomechanically induced transparency of the output field. The effects of the cavity mode decay rate on the width of the optomechanically induced transparency window, the dispersion curve slope are discussed in the resolved sideband regime and the unresolved sideband regime.

     

Quantum phase gate in decoherence-free subspace with cavity QED system

We demonstrate how to perform quantum phase gate with cavity QED system in decoherence-free subspace by using only linear optics elements and photon detectors. The qubits are encoded in the singlet state of the atoms in cavities among spatially separated nodes, and the quantum interference of polarized photons decayed from the optical cavities is used to realized the desired quantum operation among distant nodes. In comparison with previous schemes, the distinct advantage is that the gate fidelity could not only resist collective noises, but also immune from atomic spontaneous emission, cavity decay, and imperfection of the photodetectors. We also discuss the experimental feasibility of our scheme.     

Generation of W States with Many Superconducting-Quantum-Interference-Devices in Cavity QED

We propose a scheme to generate the W states with manySQUIDs (superconducting-quantum-interference-devices) in cavity QED viaRaman transition. In this scheme, the transfer of quantum informationbetween the SQUIDs and cavity is not required. And the cavity field is onlyvirtually excited, thus the cavity decay is suppressed during the W statesgeneration. The SQUIDs are always populated in the two ground states.Therefore, the scheme is insensitive to the spontaneous emission of theexcited level of the SQUID and cavity decay.     

Environmental Effects on Two-Qubit Correlation in the Dispersive Jaynes-Cummings Model

Total, classical and quantum correlations as well as entanglement are studied for a two-qubit system, where each qubit is placed in a micro cavity described by the dispersive Jaynes-Cummings model. Not only the loss of cavity photons but also the effect of the qubit-photon interaction on the loss is taken into account. The two-qubit system is initially prepared in a Bell diagonal state with a single mixing parameter and the cavity photon is either in a superposition of vacuum and single-photon states or in a weak coherent state. It is shown that more correlation of the two qubits can survive as an initial value of the cavity photon number is smaller. There is a threshold value of the cavity photon number, below which the stationary state becomes inseparable. Furthermore it is found that the external environment which causes the cavity loss has two effects; one brings about the decay of the correlations and the other suppresses the decay.     

Generation of an N-qubit phase gate via atom--cavity nonidentical coupling

A scheme for approximate generation of an N-qubit phase gate is proposed in cavity QED based on nonidentical coupling between the atoms and the cavity. The atoms interact with a highly detuned cavity-field mode, but quantum information does not transfer between the atoms and cavity field, and thus the cavity decay is negligible. The gate time does not rise with an increase in the number of qubits. With the choice of a smaller odd number l （related to atom-cavity coupling constants）, the phase gate can be generated with a higher fidelity and a higher success probability in a shorter time （the gate time is much shorter than the atomic radiative lifetime and photon lifetime）. When the number of qubits N exceeds certain small values, the fidelity and success probability rise slowly with an increase in the number of qubits N. When N→∞, the fidelity and success probability infinitely approach 1, but never exceed 1.     

Readout of a weakly coupled qubit through the use of an auxiliary mode

The dispersive coupling between a qubit and a cavity mode is widely used for performing non-destructive readout of the qubit state. In this approach, it is typically required that the dispersive strong coupling regime is achieved. Here we show that the use of an auxiliary cavity mode reduces by orders of magnitude the required value of the dispersive coupling, for a given decay rate of the cavity mode. The analysis is performed within the input-output formalism, in terms of the photon scattering matrix elements and of the signal-to-noise ratio. We derive simple analytical expressions for the optimal parameters and recover the standard single-mode result as a limiting case. The present results can also be applied to the qubit readout based on longitudinal cavity-qubit interactions, and to any sensing scheme where the cavity frequency is used as a probe to estimate some physical parameter of interest.